I cant find a Drain Plug--my inclination is to unhook the Line at the tank Prefilter, or someother line. How would a pro do this?

There's no drain plug for the fuel tank. Go under the car near the tank and loosen the hose going to the tank screen (just like you predicted).

There is indeed a drain plug on the 300D-turbo fuel tank. Remove the fuel line from the bottom of the tank that goes to the fuel pre-filter in the engine compartment. Doing that will allow you to drain the tank, and you can remove the drain plug with filter. You may want to clean the filter, or replace it.
Bill Lewallen Lexington,Ky. Where diesels purr and turbos whirr..